(a) Field of the Invention
This invention relates to cellulose based products and more particularly relates to viscose type products which can be formed into various products such as films and fibers.
(b) History of the Prior Art
The use of viscose has been used for an extended period of time in the manufacture of cellophane films, rayon fibers and other regenerated cellulose products. The viscose, dissolved modified cellulose, which has been commercially used in the prior art has almost universally been formed by treating cellulose with caustic soda and carbon disulfide to form cellulose xanthate which is then dissolved in weak caustic solution to form the viscose. The products formed from cellulose regenerated from this viscose have found great commercial success. Unfortunately, the carbon disulfide used in the process and by-product carbon disulfide and hydrogen sulfide from the process are extremely toxic and these products must be carefully managed.
In addition, in the traditional viscose process, regeneration of the cellulose is necessary. Furthermore, the resulting cellulose product does not lend itself to internal plasticization and requires some kind of plasticizer for handling. In the absence of plasticizer the product is brittle.
A viable alternate to the traditional viscose process would therefore be desirable.
As early as 1930 (U.S. Pat. No. 1,771,461) it was proposed that ammonia derivatives of carbon dioxide such as urea, could be reacted with cellulose to form soluble products which could subsequently be used for the manufacture of fibers and films. This process was further discussed in U.S. Pat. Nos. 2,129,708 (1938) and 2,134,825 (1938) assigned on their faces to E. I. du Pont. The viscose type products resulting from this process are esters which will be referred to herein as cellulose aminomethanates, although they may also be known as cellulose amino formates or cellulose carbamates in other references.
While the resulting final products, e.g. fibers and films, at least when made on a small scale, had fair properties, the properties, especially purity strength and solubility at comparable chain lengths, were not nearly as good as similar products made from conventional viscose, i.e. the xanthate process. Recently, in part due to increased awareness of our environment, interest has again been shown in the alternate viscose technology disclosed in the above early references. It has, for example, been disclosed in U.S. Pat. No. 4,404,369 issued in 1983, that an alkali-soluble cellulose derivative could be produced by treating cellulose with liquid ammonia having urea dissolved therein. The object was to develop a product having urea distributed through the product prior to reaction by heating. The process described nevertheless has problems in that liquid ammonia also must be contained and in addition the product still did not have properties as good as desired.
Various proposals have been made for increasing solubility of the cellulose aminomethanate product, e.g. U.S. Pat. No. 4,526,620 wherein excess urea is used to increase solubility but simultaneously creates urea contamination and U.S. Pat. No. 4,530,999 where the average chain length is reduced by radiation which unfortunately also decreases end product strength.
It was proposed in European Patent Publication No. 178,292 published Apr. 16, 1986, that an improved product could be obtained when the alkali-urea impregnated cellulose was washed with urea solution to remove hydroxide prior to heating to form the ester. While this provided some improvement in the properties of the resulting ester, uniformity and thus strength especially when large quantities of products were made, are still not as good as desired to permit substitution for most xanthate type viscose in most commercial applications.